Add Airtable demo

app/controllers/home_controller.rb

@@ -3,4 +3,12 @@ class HomeController < ApplicationController
   def index
     redirect_to forms_url if logged_in?
   end
+
+  def demo
+    if params[:backend] == 'airtable'
+      render 'demo-airtable'
+    else
+      render 'demo-google'
+    end
+  end
 end